A high power transformer always has some power losses during operation. It is very important to have these losses under control and monitor them frequently in order to have an early indicator of transformer problems and malfunctions. In theory it is easy to monitor the power losses by means of measuring the input power to the primary side of the transformer and measuring the output power at the secondary side of the transformer. The difference between the input power and the output power is due to transformer losses. A common measurement setup for measuring power loss involves a current sensor and a voltage sensor on both the primary side and the secondary side of the power transformer.
In reality, the measured voltage and current signals are always associated with a systematic error due to the accuracy of the measurement setup. This gives the measured voltage:V˜=V+Verr=(1+εV)·V  (eq1)
Where εV is a complex number representing the error in the voltage measurement. In a similar way, the measured current can be written as:I˜=I+Ierr=(1+εI)·I  (eq2)
Where, εI is a complex number representing the error in the current measurement. This means that a measured power loss is not equal to the true power loss, and the measured power loss is:S˜loss=V˜H·I˜H*−V˜L·I˜L*  (eq3)
By insertion of the above equations (eq1, eq2) into (eq3), an expression for the measured power loss is obtained:S˜loss=(1+εVH)(1+εIH)*VHIH* −(1+εVL)(1+εIL)*VLIL*  (eq4)
The above equation (eq4) may be used to calculate the true power loss, but there are some numerical problems associated with the above equation (eq4) due to the difference calculation. Calculation of a difference between two large numbers that are approximately equal to each other causes increased numerical errors. If the above equation for calculation of the power loss is used, some non-physical relations such as a power loss that decreases with loading and negative power loss may be observed.
EP 2 474 832 A2 discloses a method, system and computer program product for determining the health of a transformer. The method includes computing an effective turns ratio based on a primary electrical parameter associated with a primary winding of the transformer and a secondary electrical parameter associated with a secondary winding of the transformer. The method further includes computing an operational magnetizing current based on the effective turns ratio and primary and secondary currents of the transformer or primary and secondary voltages of the transformer. Finally, the method includes determining an inter-turn winding health indicator based at least in part on the operational magnetizing current.
U.S. Pat. No. 6,809,525 B1 discloses method for estimating conductor losses in a transformer having a first and a second winding includes energizing the first winding while the second winding is short-circuited by an electrical conductor so that power is supplied to the first winding and a portion of the power is dissipated due to a resistance associated with the electrical conductor. The method also includes measuring the power supplied to the first winding, calculating the portion of the power dissipated due to the resistance associated with the electrical conductor, and subtracting the portion of the power dissipated due to the resistance associated with the electrical conductor from the power supplied to the first winding.